Image forming devices including copiers, laser printers, facsimile machines, and the like, include a drum having a rigid cylindrical surface that is coated along a defined length of its outer surface with a photoconductive material. The surface of the drum is charged to a uniform electrical potential and then selectively exposed to light in a pattern corresponding to an original image. Those areas of the photoconductive surface exposed to light are discharged thus forming a latent electrostatic image on the photoconductive surface. A developer material, such as toner, having an electrical charge such that the toner is attracted to the photoconductive surface is brought into contact with the photoconductive surface. The drum then rotates past an intermediate transfer medium where the toner is transferred onto the medium. A recording sheet, such as a blank sheet of paper, is then brought into contact with the intermediate transfer medium and the toner thereon is transferred to the recording sheet in the form of the latent electrostatic image. The recording sheet is then heated thereby permanently fusing the toner to it. In preparation for the next image forming cycle, the photoconductive surface is discharged and residual toner is removed.
Typically, the toner is stored in a toner reservoir adjacent to the drum. A doctor blade and developer roller is positioned between the toner reservoir and drum for controlling the amount of toner passed to the drum. A nip created between the doctor blade and the developer roller controls the amount of toner transferred to the drum. It is important that the doctor blade make uniform and consistent contact across the entire length of the developer roller. If the doctor blade has inconsistent pressure with the developer roller during the transfer, uneven toner amounts will be transferred to the drum resulting in inconsistent and unacceptable print quality. If too much toner is transferred to the drum, printing errors may occur such as blurred images, poor color, and toner particles deposited on the background areas. Conversely, if not enough toner is transferred to the drum, the images will be too light and difficult to see.
Thus, for toner to be developed in an electro-photographic process it must be doctored out in a thin uniform layer onto a developer roll. The uniformity with which it is doctored has a direct impact on the quality of the resulting print. A problem in maintaining consistent contact and pressure is the developer roller profile may be non-uniformity requiring that the doctor blade move inward and outward to track the surface of the developer roller. Additionally, it is vital that contact be maintained across the entire length of the doctor blade to ensure even print quality across the width of the image. It is advantageous for the member doing the doctoring to be able to conform to the surface of the developer roll to reduce the effect of the geometry variations in the roll and in the doctoring member itself. It has been established that this can be accomplished using a piece of polyester coated with a conductive urethane filled with silicon carbide, attached to a foam substrate, adhered to an extruded metal bar.
One issue resulting from the use of an extruded metal bar is that the foam is unconstrained along its length and will shift with the friction of the surface of the rotating developer roll. The doctoring media (abrasively coated polyester) also has a propensity to vibrate audibly due to the stick-slip action of running against the roll. This stick-slip activity is facilitated by the shifting of the unconstrained foam which causes the doctor blade to move back and forth resulting in small perturbations on the developer roller which translate into small velocity variations. This may show up on a printed page as a repeating pattern of light and dark lines that extend across the printed image.
At least one design is known to use a conductive caulk to provide the electrical bridge between the supporting metal bar and the electrically conductive doctoring media. The caulk requires additional assembly time and curing time and suffers from a potential failure mode if applied improperly or damaged during assembly in the cartridge. The tolerances on the geometric properties of the bar limit the accuracy with which the foam can be positioned. Extensive testing has shown that positioning of the doctoring nip closer to the leading edge of the doctoring surface will result in fewer print defects such as skid marks.
One proposed solution is to lessen the amount of force that the doctor blade exerts on the developer roller. However, the amount of force applied by the doctor blade controls the amount of toner transferred to the developer roller. If the force is decreased to prevent or decrease jitter, toner transfer may be adversely affected. Also, it has been determined that lesser biasing force may prevent the doctor blade from sticking and skipping along the developer roller, but may not prevent vibrations that result in jitter. Further, another cause of jitter may be the electrical force between the developer roller and the drum. Lessening the amount of biasing force does not substantially reduce or eliminate this cause of jitter.